The invention is in the field of measuring devices, such as for measuring temperature or other parameters. A specific embodiment of the invention relates to a digital thermometer.
Prior art devices of this type typically rely on a temperature responsive element incorporated in a bridge circuit connected to a high precision voltage source. The bridge imbalance due to the temperature responsive arm is converted to a digital number by a suitable analog-to-digital converter. One disadvantage with devices of this type is that their accuracy depends on that of a high precision voltage source, which makes such devices expensive to make and difficult to maintain.
This invention is directed to an improvement in such devices which eliminates the need for a high precision voltage source. In accordance with the invention, dual slope analog-to-digital converter techniques are used in a novel way which provides an accurate measure of temperature or similar parameters without using the previously indispensable high precision voltage source.
In a specific embodiment of the invention, the input capacitor, integrator and detector of the prior art analog-to-digital converter in an autozeroing configuration is combined with a resistance bridge having a temperature responsive arm and a temperature insensitive arm. The bridge is powered by a low precision voltage source. The free side of the input capacitor is connected to the temperature insensitive bridge arm through a first switch, to the temperature responsive bridge arm through a second switch, and to the voltage source through a third switch. The other side of the input capacitor, i.e., the side which is normally connected to the integrator input, is connected to the detector output through a fourth switch in the common autozeroing configuration. By opening and closing the four switches in a sequence which reflects the invented technique, the undesirable effects of long-term changes in the voltage source level and in the offset and drift of the integrator and detector amplifiers are eliminated, and the difference between the two arms of the bridge is measured accurately, thus obtaining an accurate measure of the effect of temperature on the temperature responsive bridge arm.
Specifically, only the second and fourth switches are initially in a closed state, to thereby place on the input capacitor an initial charge which reflects: (a) the voltage level of the voltage source, (b) the resistance of the temperature responsive bridge arm, and (c) the offset and drift of the integrator and detector amplifiers. Then, a dual slope measurement cycle is started comprising a fixed time interval and a variable time interval. During the fixed time interval, only the first switch is maintained in a closed state, all other switches being open, to thereby apply to the integrator a measurement voltage which reflects: (a) the initial charge on the input capacitor, (b) the voltage level of the voltage source, and (c) the resistance of the temperature insensitive bridge arm. During the variable time interval, only the third switch is maintained in a closed state, all other switches being open, to thereby apply to the integrator a reference voltage whose polarity is opposite that of the measurement voltage and whose magnitude reflects the voltage level of the voltage source. The variable time interval ends when the output of the integrator crosses the reference voltage. The duration of this variable time interval is a measure of the difference between the resistances of the two bridge arms, and is therefore a measure of the temperature of interest. If the levels of the voltage source and of the drift and offset of the integrator and detector amplifiers remain reasonably constant during the dual slope measurement cycle, the absolute values of these levels or changes in these levels as between successive measurement cycles have no effect on accuracy. Since each measurement cycle is of the order of milliseconds, it is unlikely that the level of the voltage source or the level of the drift and offset of the amplifiers would change during a measurement cycle. The longer term changes, as between measurement cycles, do not affect accuracy.